JPH09209099A - Production of seamless tube made of alpha plus beta titanium alloy - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of producing a seamless tube made of alpha plus beta titanium alloy, minimal in material anisotropy and having excellent mechanical properties. SOLUTION: The seamless tube composed of alpha plus beta titanium alloy is produced on the piercing system. In this method, a solid billet is heated to a temp. not lower than the β-transformation point of the alloy and lower than β-transformation point +400 deg.C) and formed into tube. Then, the tube is heated and held at a temp. not higher than the β-transformation point and not lower than the temp. at which the volume ratio between the alpha and the beta phase of this alloy becomes 1:1 in equilibrium state for >=20min. Subsequently, the tube is subjected to a first heat treatment consisting of cooling at a cooling velocity not lower than air cooling velocity and then to a second heat treatment consisting of holding at a temp. not lower than 550 deg.C and not higher than the temp. at which the volume ratio between the alpha and the beta phase of this alloy becomes 7:3 in equilibrium state for 20min to <2hr.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a seamless pipe made of α + β type titanium alloy.

[0002]

2. Description of the Related Art Titanium alloys are lightweight, have high strength, and have high corrosion resistance, so they can be used in the extreme environment of deep depth, high temperature, and high corrosion in recent years, such as geothermal development, submarine oil field and gas field development. Is being watched as. In particular, α + β type titanium alloys, which have a proven track record in aircraft and the like, and highly corrosion resistant α + β type titanium alloys in which a small amount of Pd or Ru is added to the titanium alloys to further enhance the corrosion resistance, are regarded as promising materials for the extreme environment. In the above applications, the pipe is the main product shape, and the titanium alloy pipe material is manufactured by bending and welding the plate (welded pipe), hot extrusion (seamless pipe), and plug mill. A method of making a pipe by piercing / rolling (seamless pipe) can be considered. Of these, the drilling and rolling method using a plug mill has the highest yield,
Further, since the production efficiency is high, this is a particularly advantageous method for a titanium alloy whose material itself is already expensive. In addition, it is also suitable for the above-mentioned extreme environmental use that a seamless pipe having no welded portion, which may cause deterioration of characteristics, can be manufactured.

When a seamless tube made of α + β type titanium alloy is produced by this method, the α + β type alloy has a remarkably high deformation resistance in a temperature range below the β transformation point and also has a poor hot ductility. Most of the hot working must be done in the β single-phase region. However, in general, if the strain is not sufficiently released by performing such strong working in the β single-phase region, α that precipitates during cooling at a temperature below the β transformation point
The crystal orientation of the phase preferentially has a specific orientation, and as a result, remarkable material anisotropy occurs. This anisotropy is cooled to the α + β two-phase temperature range in the latter stage of hot working, and even if a small amount of working is added there, it is hardly resolved, and a strong anisotropy still remains.

To eliminate such strong anisotropy,
High-pressure processing is performed in the α + β region, and the rolling direction is 90
The so-called cross rolling is carried out by rotating at a temperature of °, the hot working is completed in the high temperature region of the β single phase region, plastic strain in the β phase is eliminated while cooling to the β transformation point, and thereafter at the β transformation point or lower. Prevent the crystallographic orientation of the precipitated α phase from being specified,
The material after hot working is heated once to a temperature above the β transformation point, new β crystal grains without plastic strain are generated, and then cooled, so that the crystal orientation of the precipitated α phase will not be specified. It is possible to do so. However, is α +
It requires strong working in the β region and cannot be applied to the drilling / rolling method using a plug mill. In the case of a plate, cross rolling is possible, but this is also difficult with a plug mill.
And are simultaneously released from the strain in the β phase,
Since the crystal grains are coarsened, there is a drawback that mechanical properties after cooling, particularly ductility becomes poor.

[0005]

The present invention solves the above problems and provides a method for producing an α + β type titanium alloy seamless pipe having a small material anisotropy and excellent mechanical properties. The purpose is to do.

[0006]

The gist of the present invention is as follows. (1) In the method of manufacturing a seamless pipe made of α + β type titanium alloy by a piercing / rolling method, a solid billet is heated to a temperature not lower than β transformation point of the alloy and lower than β transformation point + 400 ° C. to form a pipe. Then, after heating and holding for 20 minutes or more at a temperature at which the volume ratio of the α phase and the β phase of the alloy is 1: 1 in parallel at the β transformation point or less, cooling is performed at a cooling rate of air cooling or more. And then a second heat treatment at a temperature of 550 ° C. or higher and a temperature ratio of 7: 3 or less in a parallel state of the α phase and β phase of the alloy for 20 minutes or more and less than 2 hours. A method for producing an α + β type titanium alloy seamless tube, characterized by: (2) The α + β type titanium alloy contains oxygen + nitrogen in a total of 0.
25% by weight or more is contained, The manufacturing method of the seamless tube made from an α + β type titanium alloy according to (1).

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION An α + β type titanium alloy is a martensitic transformation partly or entirely when quenching from the β single phase temperature range with the α + β two phase as the main phase in an equilibrium state at room temperature. Ti-6Al-4, a type of alloy
V, Ti-6Al-6V-2Sn, Ti-6Al-2S
n-4Zr-6Mo, Ti-6Al-1.7Fe-0.
2Si, Ti-5.5Al-1Fe-0.15 oxygen-
0.05 nitrogen, Ti-5Al-2.5Fe, etc. correspond to this. In addition, alloys such as Ti-6Al-4V-0.2% Pd in which platinum group elements such as Pd and Ru are further added to improve corrosion resistance and T in which the amount of interstitial impurity elements are reduced.
i-6Al-4V-ELI (Extra-Low Interstitials
) Etc. also belong to α + β type titanium alloys. These α + β
In the equilibrium state, titanium alloys have FeTi phase, ω phase,
Silicide, Ti-Al-based ordered phase, Ti-O-based ordered phase,
Some of them contain a Ti-N-based ordered phase, an intermetallic compound phase, etc., but are substantially α + β in the temperature range below the β transformation point.
The volume fraction of the α phase is 0 above the β transformation point, and at temperatures below that, α decreases with decreasing temperature.
The proportion of phases increases, and at room temperature, it is composed of approximately 75 to 95% of α phase and the balance β phase, although it depends on the alloy species.

In the method for producing such an α + β type titanium alloy seamless tube by the piercing / rolling method, first of all, according to the present invention, a solid billet is formed of β of the alloy.
It was decided to heat to a temperature not lower than the β transformation point and 400 ° C. above the transformation point to perform pipe forming. Here, the pipe forming refers to a hot working process, and refers to a series of rolling working processes such as drilling and stretching, polishing, shaping and drawing.

The reason for heating above the β transformation point is that
For the following reasons. That is, the large amount of processing in the pipe forming step is the step of piercing and stretching which is the initial processing step, and this step has a β single phase region above the β transformation point with low hot deformation resistance and high hot ductility. Because it is necessary to do it in. In this process, since the amount of processing is large, heat is easily generated during processing, and a rapid temperature change does not occur. Therefore, if the billet heating temperature is set to β transformation point or higher, most or all of the processing has excellent workability. It can be performed in the β single-phase region. Since the amount of processing is small in the subsequent processing steps after the stretching, it is possible to perform the processing in the α + β region where the deformation resistance is high and the hot ductility is poor. The heating temperature is set to a temperature lower than the β transformation point + 400 ° C., because if it is heated to a temperature higher than this, slippage due to the generated oxide scale becomes severe and pipe forming becomes impossible.

Next, after the hot-worked tube is heated and held at a temperature not higher than the β transformation point for at least 20 minutes at a temperature at which the volume ratio of the α phase to the β phase of the alloy is 1: 1. Then, a first heat treatment for cooling at a cooling rate higher than air cooling is performed. This is α
By heating to a high temperature in the + β region, most of the α phases oriented in the specific crystal orientation, which is the cause of the anisotropy, are transformed into the β phase, and in the process for releasing the plastic strain in the β phase. is there. Here, the heating temperature is set to be the β transformation temperature or lower and the temperature at which the volume ratio of the α phase and the β phase of the alloy becomes 1: 1 in the equilibrium state, because the heating temperature is higher than the β transformation point. This is because grains are coarsened and mechanical properties, especially ductility are deteriorated, and even if the alloy is heated below a temperature at which the volume ratio of α phase and β phase is 1: 1 in an equilibrium state, This is because most of the α phase accumulated in the azimuth remains as it is, and the effect of reducing anisotropy does not appear so much, and the effect of the present invention becomes insufficient. Further, heating for 20 minutes or more was required because heating for less than 20 minutes resulted in insufficient transformation from α phase to β phase, which was far from the equilibrium state and azimuthally integrated in a specific orientation. This is because the α phase was not sufficiently eliminated. Here, the upper limit of the heating time is not particularly specified, but this is because the grain growth rate of both the α phase and the β phase is slow during heating in the α + β two-phase region, and the holding time is longer than 10 hours. Because there is no big change in the organization. However, from the viewpoint of energy, and from the viewpoint of oxidative loss in the case of atmospheric oxidation, it is desirable to heat for a suitable time according to the size of the tube.

In this step, the cooling is limited to the air cooling or more, and the reason is as follows. That is, when cooled at a cooling rate higher than air cooling, the volume fraction of the α phase azimuthally integrated in a specific orientation is frozen to room temperature as it is,
The phase transforms into a fine martensitic structure, or a fine acicular α phase having a random orientation in which orientation is not accumulated in a specific orientation is precipitated, or the β phase is frozen to room temperature in a metastable state. Which of these changes occurs depends on the alloy component and the heating temperature (cooling start temperature), but as far as the present invention is concerned, it does not matter what kind of change occurs. Done α
The point is to reduce the phase so that it does not increase again during cooling. If cooled at a slower cooling rate than air cooling, the β phase will undergo any of the above-mentioned structural changes during cooling, namely transformation into a martensitic structure, precipitation of randomly oriented fine acicular α phase, and freezing of metastable β phase. The effect of the heat treatment is that the α phase accumulated in a specific orientation that has remained grows again during cooling and eventually returns to the state with strong anisotropy before heat treatment, although it does not change Disappears.

Now, following the first heat treatment, 550
A second heat treatment is performed at a temperature of not less than 0 ° C. and not more than 20 minutes and less than 2 hours at a temperature at which the volume ratio of α phase and β phase of the alloy is 7: 3 in a parallel state. The purpose of this second heat treatment is to stabilize the non-equilibrium unstable structure produced by cooling at a cooling rate higher than air cooling, which is the final step of the first heat treatment. At this second heat treatment temperature, the equilibrium α
Since the volume fraction of the phase is larger than the volume fraction of the α phase at the first heat treatment temperature, the volume fraction of the α phase increases during the second heat treatment.

This increase is brought about by the following tissue changes. That is, the structure transformed into martensite in the first heat treatment is decomposed into fine acicular α-phase and β-phase,
At this time, the crystal orientation of the α phase is random. In addition, the fine acicular α-phase having a random orientation precipitated during the cooling of the first heat treatment is stabilized through some growth and distribution of alloying elements. Further, in the metastable β phase frozen to the room temperature by cooling in the first heat treatment, fine acicular α phase with random crystal orientation is precipitated. Through any of these changes,
The crystal orientation of the newly generated α phase is random and is not integrated in a specific crystal orientation. Therefore, the material anisotropy of the tube immediately after hot working is reduced. Moreover, the structure finally obtained through the process of the present invention has a relatively coarse α-phase (primary α-phase) which has not disappeared in the first heat treatment and a fine needle-like α-phase (secondary α-phase) formed between them (secondary α-phase). The average strength in the length direction and the circumferential direction is equal to or higher than that of a usual annealed material due to the effect of a fine secondary α phase.

Here, the heating temperature of the second heat treatment is 55
The reason why the temperature at which the volume ratio of the α phase to the β phase of the alloy is 7: 3 in the parallel state at 0 ° C. or higher is set to be equal to or lower than the temperature is 0 or less.
That is, at a temperature lower than 550 ° C., martensite is decomposed or secondary α phase generated by precipitation in the frozen β phase is too fine and ductility is reduced. Even if the fine acicular α phase was generated during the cooling of the first heat treatment, when the heat treatment was performed at a temperature of less than 550 ° C., the acicular α
The phase does not grow slightly, but the ultrafine secondary α-phase precipitates in the β-phase existing between the acicular α-phases, leading to a decrease in ductility. As described above, in any case, performing the second heat treatment at a temperature lower than 550 ° C. causes a decrease in ductility. Further, when the volume ratio of the α phase and the β phase of the alloy is 7: 3 or more in a parallel state, the amount of the secondary α phase in the random orientation is small and the effect of improving the anisotropy becomes small. Is.

Further, the time of the second heat treatment is 20 minutes or more 2
The reason why the time is less than 20 minutes is that the diffusion of the elements is insufficient in less than 20 minutes, and a sufficiently stable structure close to the equilibrium state cannot be obtained. When heat treatment is performed for 2 hours or more, the stable structure already formed This is because changes occur as much as possible to become an extremely stable organization. The change is a phenomenon in which the primary α phase begins to erode the secondary α phase and begins to grow, aiming at the interface between the α phase and the β phase and the structure with few crystal grain boundaries between the α phases. When this happens, the proportion of α phase accumulated in a specific orientation increases and the anisotropy becomes stronger again, and the proportion of secondary α phase that has contributed to the increase in strength decreases, so the strength of the material also tends to decrease. Appears.

As described above, when the solid billet is heated and formed into a pipe within the range of temperature, time, cooling rate, etc. specified in the present invention, the material anisotropy is reduced. It is possible to manufacture an α + β type titanium alloy seamless tube having excellent mechanical properties. If a small amount of cold working such as cold straightening and cutting is performed between the first heat treatment and the second heat treatment, it is convenient because the second heat treatment also serves as strain relief annealing.

Now, in the α + β type titanium alloy, the production method of the present invention can be advantageously carried out by adding oxygen and nitrogen in a total amount of 0.25% by weight or more. That is, the addition of oxygen and nitrogen has the effect of moderating the change in the volume ratio of the α phase and the β phase at a temperature below the β transformation point, and in particular, when 0.25 wt% or more of oxygen + nitrogen is contained, The temperature range of the first heat treatment remarkably expands, and the first heat treatment becomes easy to perform. Ti-6Al-4V- with the oxygen and nitrogen contents of Ti-6Al-4V set higher
X [O + N] alloy (X is 0.25 wt% or more) and Ti-1.5Fe- with oxygen and nitrogen added as main elements.
This corresponds to 0.5% oxygen-0.04% nitrogen.

[0018]

【Example】

[Test 1] Ti-6Al-4V by vacuum arc melting
(Oxygen: 0.18% by weight, nitrogen: 0.03% by weight), T
i-6Al-4V-ELI (oxygen: 0.10 wt%, nitrogen: 0.02 wt%), Ti-6Al-4V-0.29
[O + N] (oxygen: 0.19 wt%, nitrogen: 0.10 wt%), Ti-6Al-4V-ELI-0.1Pd (oxygen: 0.10 wt%, nitrogen: 0.02 wt%, Pd :
0.10% by weight) of the four alloys were melted. FIG. 1 shows the temperature dependence of the volume fractions of α phase and β phase of the above four kinds of alloys.
Table 1 shows the β transformation point, the temperature at which the volume fraction of α phase and β phase becomes 1: 1 and the temperature at which the volume fraction of α phase and β phase become 7: 3, which are read from FIG. . Here, Ti-6Al
-4V-ELI and Ti-6Al-4V-ELI-0.1
Almost no difference was detected between Pd, and the temperature dependence of the volume fractions of α phase and β phase was the same in both.

[0019]

[Table 1]

These ingots are 215 mm × by slab rolling.
A solid billet having a square cross section of 215 mm was formed into a pipe having an inner diameter of 150 mm and a wall thickness of 20 mm by a plug mill method, and further cut into a length of 50 cm and heat-treated. Then, in parallel with the circumferential direction and the length direction, a round bar test piece having a score distance of 30 mm and a diameter between scores of 6.25 mm was cut out and subjected to a tensile test.
The billet heating conditions, heat treatment conditions, and tensile test results are shown in Tables 2, 3, 4, and 5.

[0021]

[Table 2]

[0022]

[Table 3]

[0023]

[Table 4]

[0024]

[Table 5]

Table 2 shows the conditions and results of the tests conducted on Ti-6Al-4V. Test number 1 is a conventional example in which a billet was heated immediately above the β transformation point, pipe-formed, and ordinary annealing was performed. The tensile properties in the length direction have a high elongation of 10% or more, but the tensile strength is a low value of 950 MPa or less, while the tensile properties in the circumferential direction are a tensile strength of a high value of 1000 MPa or more. However, the elongation is a low value of less than 8%, and strong anisotropy occurs in the length direction and the circumferential direction. The difference in tensile strength between the circumferential and longitudinal directions is 1
It is over 00 MPa.

On the other hand, in Test Nos. 2, 4, 5, 8, 9, 11, and 13 which are examples of the present invention, the difference between the tensile strengths in the length direction and the circumferential direction was reduced to 70 MPa or less. ing. In addition, the tensile properties in the length direction are all 95
It has a high tensile strength of 0 MPa or more and an elongation value of 10% or more, and the tensile properties in the circumferential direction also have a high tensile strength of 1000 MPa or more and a high elongation value of 8% or more. In addition to being small, high strength and ductility are obtained.

On the other hand, in the test No. 3 which is a comparative example, the heating temperature of the billet was less than the β transformation point which is the lower limit value in the present invention, so the α phase was precipitated and the deformation resistance became high, and hot working could not be performed. It was Further, in Test No. 6, since the billet heating temperature exceeded the upper limit value in the present invention, slippage due to the oxide scale was severe and pipe forming was also impossible.

In Test No. 7, since the heating temperature of the first heat treatment exceeded the β transformation point, which is the upper limit of the present invention, the anisotropy was resolved, but the β grains became coarse and the mechanical properties, especially ductility, were increased. Has deteriorated. Further, in Test Nos. 10, 12, and 14, the difference in tensile strength between the length direction and the circumferential direction is 90 MPa or more, which has strong anisotropy, and the strength in the length direction is 950.
It has a low value of not more than MPa and an elongation in the circumferential direction of not more than 8%. The reason for this is that in Test No. 10, the heating temperature of the first heat treatment was less than the lower limit of the present invention, and in Test No. 12, the holding time of the first heat treatment was shorter than the lower limit of the present invention. This is because, in Test No. 14, the cooling rate after the first heat treatment was not the air cooling specified by the present invention but the furnace cooling.

Table 3 shows the conditions and results of the tests conducted on Ti-6Al-4V-ELI. Exam number 15
Is a conventional example in which a billet was heated to a β transformation point or higher, pipe-formed, and ordinary annealing was performed. Tensile property in the length direction is 12
Although it has a high elongation of more than 100%, the tensile strength is 900
The tensile strength in the circumferential direction is a high value of 950 MPa or more, but the elongation is a low value of less than 10%, and it is strong in the length direction and the circumferential direction. Anisotropy is occurring. The difference in tensile strength between the circumferential direction and the longitudinal direction is 100 MPa or more.

On the other hand, in Test Nos. 17 and 18 which are examples of the present invention, the difference between the tensile strengths in the length direction and the circumferential direction is reduced to 60 MPa or less. In addition, the tensile properties in the length direction each have a high tensile strength of 900 MPa or more and a high elongation value of 12% or more, and the tensile properties in the circumferential direction also have a high tensile strength of 950 MPa or more and 10%.
It has the above-mentioned high elongation value, and the material anisotropy is small, and high strength and ductility are obtained.

On the other hand, in Test No. 16, since the heating temperature of the first heat treatment exceeded the β transformation point which is the upper limit of the present invention, the anisotropy was resolved, but the β grains became coarse and the mechanical properties, Especially the ductility deteriorated. Further, Test No. 19 has a strong anisotropy with a difference in tensile strength between the longitudinal direction and the circumferential direction of 100 MPa and has a strong anisotropy of 900 MPa or less and an elongation of 10% or less in the circumferential direction. It is a low value. The reason for this is that the heating temperature of the first heat treatment was less than the lower limit of the present invention.

Table 4 shows Ti-6Al-4V-0.29.
It is a condition of the test conducted on [O + N] and its result. Test number 20 is a comparative example in which the billet was heated to a β transformation point or higher, pipe-formed, and ordinary annealing was performed. The tensile property in the length direction has a high elongation of 9% or more, but the tensile strength is 1050 MPa or less, which is low relative to the high concentration of oxygen and nitrogen, while the tensile strength in the circumferential direction is low. The tensile properties have a high tensile strength of 1100 MPa or more, but a low elongation value of less than 7%, and strong anisotropy occurs in the length direction and the circumferential direction. The difference in tensile strength between the circumferential direction and the longitudinal direction is 100 MPa or more.

On the other hand, in Test Nos. 22, 23, and 24, which are examples of the present invention, the difference in tensile strength between the length direction and the circumferential direction is reduced to 60 MPa or less. Further, the tensile properties in the length direction each have a high tensile strength of 1050 MPa or more and a high elongation value of 9% or more, and the tensile properties in the circumferential direction are also a high tensile strength of 1100 MPa or more and a high 8% or more. It has an elongation value, has a small material anisotropy, and has high strength and ductility.

On the other hand, in Test No. 21, since the heating temperature of the first heat treatment exceeded the β transformation point which is the upper limit value of the present invention, the anisotropy was resolved, but the β grains became coarse and the mechanical properties, Especially the ductility deteriorated. Further, Test No. 25 has a strong anisotropy with a difference in tensile strength between the lengthwise direction and the circumferential direction of 100 MPa, and has a strength in the lengthwise direction of 1050 MPa or less,
The elongation in the circumferential direction is as low as 8% or less. The reason for this is that the heating temperature of the first heat treatment was less than the lower limit of the present invention.

As shown in Table 1, the total amount of oxygen and nitrogen is 0.21% by weight for Ti-6Al-4V,
-6Al-4V-ELI 0.12 wt%, Ti-6
The amount of Al-4V-0.29 [O + N] is 0.29% by weight. Further, as shown in FIG. 1 and Table 1, the first aspect of the present invention
The temperature range of the heat treatment of Ti-6Al-4V is 925-925.
65 ° C. between 990 ° C., 60 ° C. between 910 and 970 ° C. for Ti-6Al-4V-ELI, Ti-6Al-4V-
In the case of 0.29 [O + N], 105 between 935 and 1040 ° C
℃, and Ti containing 0.25 wt% or more of oxygen + nitrogen
In -6Al-4V-0.29 [O + N], the first heat treatment temperature range is clearly widened, and in fact, the effect of the present invention appears in this wide temperature range.
2, 23, 24.

Table 5 shows Ti-6Al-4V-0.1Pd.
It is the conditions and the results of the test conducted on. Test No. 26 is a conventional example in which the billet was heated to a β transformation point or higher, pipe-formed, and annealed. 12% tensile property in the length direction
Although it has the above high elongation, the tensile strength is 900M.
It is a low value below Pa, while the tensile properties in the circumferential direction are
Tensile strength is as high as 950 MPa or more, but elongation is 1
It is a low value of less than 0%, and strong anisotropy occurs in the length direction and the circumferential direction. The difference in tensile strength between the circumferential direction and the longitudinal direction is 100 MPa or more.

On the other hand, in the test numbers 28, 29, 30, 31, 34, and 35, which are the examples of the present invention, the difference in tensile strength between the length direction and the circumferential direction is reduced to 50 MPa or less. . In addition, the tensile properties in the longitudinal direction each have a high tensile strength of 900 MPa or more and a high elongation value of 12% or more, and the tensile properties in the circumferential direction are also high tensile properties of 950 MPa or more and high 10% or more. It has an elongation value, has a small material anisotropy, and has high strength and ductility.

On the other hand, in Test No. 27, since the heating temperature in the second heat treatment exceeded the upper limit of the present invention, the effect of reducing anisotropy was small, and the strength in the length direction was 900 MPa.
The value was as low as a or less, and the elongation in the circumferential direction was also as low as less than 10%. Further, in the test number 32, the heating temperature of the second heat treatment was too low, so the ductility decreased in both the length direction and the circumferential direction (elongation value of less than 10%), and in the test number 33, the second heat treatment was maintained. Since the time was too short, the structure was not stabilized and the ductility deteriorated. In Test No. 36, the second heat treatment was performed in excess of the upper limit of the present invention, so that the difference in tensile strength between the circumferential direction and the length direction was a large value of 80 MPa or more, and the strength in the length direction was large. Is 900MP
The elongation was a or less, and the elongation in the circumferential direction was as low as 10% or less.

[Test 2] Ti-1.5 having a square cross section of 215 mm × 215 mm was obtained by lump rolling after vacuum arc welding.
Wt% Fe-0.5 wt% oxygen-0.04 wt% nitrogen alloy (hereinafter Ti-1.5Fe-0.5 [O] -0.04
([N]), a solid billet is produced, and a plug mill method is used to form an inner diameter of 150 mm and a wall thickness of 20 mm.
After cutting to a length of 0 cm, heat treatment was performed. The temperature dependence of the volume fraction of α phase and β phase of this alloy is as shown in FIG. 2, the β transformation point is 965 ° C., and the temperature at which the volume ratio of α phase and β phase is 1: 1 is 820 ° C. The temperature at which the volume ratio of the α phase to the β phase becomes 7: 3 is 755 ° C. Oxygen + nitrogen is 0.25 in this alloy
0.54% by weight, which is much higher than the weight%, is added. Therefore, the heating temperature range of the first heat treatment is 820 to 20%.
It is 145 ° C. between 965 ° C., which is wider than the example shown in the test 1. Distance from this tube parallel to the circumferential and length directions, scoring distance 30 mm, diameter between grading 6.
A 25 mm round bar test piece was cut out and subjected to a tensile test. Table 6 shows the billet heating conditions, heat treatment conditions, and tensile test results.
As shown in FIG.

[0040]

[Table 6]

In Table 6, Test No. 37 is a comparative example in which the billet was heated to a β transformation point or higher, piped, and annealed. The tensile property in the length direction has a high elongation of 15% or more, but the tensile strength is a low value of 950 MPa or less, while the tensile property in the circumferential direction is 10% or less.
Although it is a high value of 00 MPa or more, the elongation is a low value of less than 10%, and strong anisotropy occurs in the length direction and the circumferential direction. The difference in tensile strength between the circumferential and longitudinal directions is 100
It is higher than MPa.

On the other hand, test numbers 38, 40, 42, 43, 45, 47, 51, 53, which are examples of the present invention,
In No. 54, the difference in tensile strength between the length direction and the circumferential direction is reduced to 70 MPa or less. In addition, the tensile properties in the length direction each have a high tensile strength of 950 MPa or more and a high elongation value of 15% or more, and the tensile properties in the circumferential direction also have a high tensile strength of 1000 MPa or more and 10% or more. It has a high elongation value, has a small material anisotropy, and has high strength and ductility.

On the other hand, in Test No. 39, which is a comparative example, the heating temperature of the billet was less than the β transformation point, which is the lower limit of the present invention, so the α phase was precipitated and the deformation resistance increased,
Hot working could not be done.

In Test No. 41, since the heating temperature of the first heat treatment exceeded the β transformation point, which is the upper limit of the present invention, the anisotropy was resolved, but the β grains became coarse and the mechanical properties, especially ductility were increased. Has deteriorated. Further, in Test Nos. 44, 46, and 48, the difference in tensile strength between the longitudinal direction and the circumferential direction is 90 MPa or more, which has strong anisotropy, and the strength in the longitudinal direction is 95.
It has a low value of 0 MPa or less and a circumferential elongation of 10% or less. The reason for this is that in Test No. 44, the heating temperature of the first heat treatment was less than the lower limit value of the present invention,
This is because the holding time of the first heat treatment was shorter than the lower limit value of the present invention in Test No. 46, and the cooling rate after the first heat treatment in Test No. 48 was not equal to or higher than the air cooling specified in the present invention, Because it was there.

In Test No. 49, the heating temperature in the second heat treatment exceeded the upper limit of the present invention, so the effect of eliminating anisotropy was small, and the strength in the longitudinal direction was a low value of 950 MPa or less, and The elongation was also as low as less than 10%. Further, in the test number 55, since the heating temperature of the second heat treatment was too low, the ductility was deteriorated in both the length direction and the circumferential direction, and values of less than 15% in the length direction and less than 10% in the circumferential direction were obtained. There wasn't. In Test No. 50, the holding time of the second heat treatment was too short, so the structure was not stabilized and the ductility decreased. In Test No. 52, the second heat treatment was performed in excess of the upper limit of the present invention, so that the difference in tensile strength between the circumferential direction and the length direction was a large value of 70 MPa or more, and the strength in the length direction was 950 MPa. Hereinafter, the elongation in the circumferential direction was a low value of 10% or less.

[0046]

As described above, according to the present invention, an α + β type titanium alloy seamless tube having a small material anisotropy and excellent mechanical properties can be manufactured.

[Brief description of drawings]

FIG. 1 Ti-6Al-4V, Ti-6Al-4V-E
LI, Ti-6Al-4V-0.29 [O + N], Ti
It is a figure which shows the temperature dependence of the volume fraction of (beta) phase and (alpha) phase in the parallel state of -6Al-4V-ELI-0.1Pd. Here, Ti-6Al-4V-ELI and Ti-6A
No significant difference was observed for l-4V-ELI-0.1Pd, so both are shown in the same curve.

FIG. 2 Ti-1.5Fe-0.5 [O] -0.04
It is a figure which shows the temperature dependence of the volume fraction of (beta) phase and (alpha) phase in the parallel state of [N].

Claims (2)

[Claims] 1. A method for producing a seamless tube made of an α + β type titanium alloy by a piercing / rolling method, wherein a solid billet has a β transformation point of +400 or more at a β transformation point of the alloy or more.
The pipe is heated to a temperature below ℃, and then the volume ratio of the α phase and β phase of the alloy is 1: 1 at the β transformation point or lower.
After heating for 20 minutes or more at a temperature higher than or equal to the above temperature, a first heat treatment of cooling at a cooling rate of air cooling or more is performed, and then 55
An α + β type characterized by performing a second heat treatment of holding the temperature at 20 ° C. or higher for less than 2 hours at a temperature of 0 ° C. or higher and at a temperature at which the volume ratio of α phase and β phase of the alloy is 7: 3 in a parallel state. Titanium alloy seamless tube manufacturing method. 2. The method for producing an α + β type titanium alloy seamless tube according to claim 1, wherein the α + β type titanium alloy contains 0.25% by weight or more of oxygen + nitrogen in total.